Electrical energy storage devices, such as capacitors, batteries, and ultracapacitors, store or create energy by utilizing the electric charge on two metal or otherwise electrically conductive surfaces (“electrodes”). The charge-bearing surfaces are typically separated by an electrical insulator, or dielectric. As charge is placed on the conductive surfaces, an electrical field is established between the electrodes, resulting in a voltage. Typically, a capacitor physically separates positive and negative charges, rather than chemically separating the charges, as is common in batteries. Batteries have limited ability to be recycled and cannot deliver energy as quickly as a capacitor, or without greater losses than occurs with capacitors.
A supercapacitor or ultracapacitor is sometimes called a double-layer capacitor, as it polarizes an electrolytic solution to store energy electrostatically. The energy storage mechanism of an ultracapacitor is highly reversible, which allows for the ultracapacitor to be charged and discharged many times.
However, capacitors typically have not been able to match the energy storage capability of batteries due to the lack of available materials and structures that can tolerate electric fields of sufficient strength.